Conventional gate dielectrics used in semiconductor memory devices consist of a thin SiO.sub.2 layer. A recent trend in semiconductor processing has been the inclusion of a small concentration of nitrogen in the gate dielectric layer. It has been found that the nitrogen provides the beneficial effects of reducing channel hot electron damage and reducing boron out diffusion from the polysilicon gate into the channel. The inclusion of nitrogen further raises the dielectric constant so that a nitrided film has a lower leakage current than a pure oxide film that has an equivalent capacitance. Even though nitrogen has beneficial effects on gate insulators, too large a nitrogen concentration may be undesirable. Large nitrogen concentrations can cause unacceptable shifts in Vfb, as well as degradation in other properties of the dielectric.
There is little flexibility in the control of the concentration and depth distribution of nitrogen in most methods for introducing nitrogen into SiO.sub.2. Chemical methods of introducing nitrogen rely on the reaction between a nitriding agent such as NO or NH.sub.3 with the silicon substrate or a previously grown oxide. The resulting films have a large concentration of nitrogen at the Si/SiO.sub.2 interface where the chemical reaction takes place. The reaction is self-limiting by the nitrogen since the nitrided layer acts as a diffusion barrier to oxygen and thus prevents further gas species from reaching the Si/SiO.sub.2 interface. This provides an additional benefit of using nitrogen in a nitriding process, i.e., the nitrided films are more uniform in thickness than conventional oxides. The uniformity in thickness is demonstrated by the smaller distribution of electrical characteristics measured at different sites across a wafer surface treated with, for example, NO oxidation.
Some limited control of the nitrogen depth distribution has been attempted by others. One of the methods is to control the initial nitridation conditions. For example, it has been shown that the nitrogen content in an oxynitride layer created by exposing silicon to gaseous NO depends on the nitridation temperature. By reacting at a lower temperature, a smaller quantity of nitrogen is introduced, even though the quality of the resulting dielectric may be compromised by the lower reaction temperature. Another method for controlling the nitrogen depth distribution is by reoxidation of oxynitrides. For instance, it has been shown that a pure SiO.sub.2 spacer layer can be inserted in-between an oxynitride layer and a silicon substrate by exposing the sample to gaseous O.sub.2 at elevated temperatures. The oxygen diffuses through the dielectric and reacts with the silicon substrate to form the underlying SiO.sub.2 layer without disturbing the oxynitride film. It was also shown that nitrogen can be removed upon reoxidation by N.sub.2 O, even though it may be more desirable to leave a controlled amount of nitrogen in the oxynitride layer instead of the complete removal such that desirable benefits of nitrogen may be retained. It is therefore desirable to have a method that effectively controls the profile of nitrogen concentration in an oxynitride layer used as a gate dielectric, while simultaneously, after a reoxidation process of the oxynitride layer is carried out, forming a substantially pure SiO.sub.2 layer underneath the dielectric.
It is therefore an object of the present invention to provide a method for forming an oxynitride gate dielectric in a semiconductor device that does not have the drawbacks or shortcomings of the conventional methods.
It is another object of the present invention to provide a method for forming an oxynitride gate dielectric in a semiconductor device that is capable of achieving a controlled profile of nitrogen concentration in the oxynitride.
It is a further object of the present invention to provide a method for forming an oxynitride gate dielectric in a semiconductor device that is capable of producing, after a re-oxidation to process of oxynitride, a substantially pure SiO.sub.2 layer underneath the dielectric.
It is another further object of the present invention to provide a method for forming an oxynitride gate dielectric in a semiconductor device which has a controlled nitrogen profile in the oxynitride layer by first forming the layer by contacting a surface of silicon with at least one gas that contains nitrogen and/or oxygen.
It is still another object of the present invention to provide a method for forming an oxynitride gate dielectric in a semiconductor device that has a controlled profile of nitrogen concentration by first forming the oxynitride layer on a silicon surface by a chemical vapor deposition technique.
It is yet another object of the present invention to provide a method for forming an oxynitride gate dielectric in a semiconductor device by first forming an oxynitride layer and then treating the layer with a gas mixture comprising oxygen and at least one halogenated species for forming a substantially silicon dioxide layer underneath the oxynitride layer.
It is another further object of the present invention to provide a method for forming an oxynitride gate dielectric in a semiconductor device by contacting a surface of silicon with at least one gas that contains nitrogen and/or oxygen selected from the group consisting of NO, N.sub.2 O, NH.sub.3 and O.sub.2.
It is yet another further object of the present invention to provide a gate dielectric situated in a semiconductor device that includes a spacer layer of substantially SiO.sub.2 overlying a silicon substrate, an oxynitride layer overlying the spacer layer and a SiO.sub.2 layer overlying the oxynitride layer.
It is still another further object of the present invention to provide a gate stack situated in a semiconductor memory device that includes a spacer layer of substantially SiO.sub.2 overlying a silicon substrate, an oxynitride layer overlying the spacer layer, a silicon dioxide layer overlying the oxynitride layer and a conductive gate overlying the silicon dioxide layer.